The popularity of multimedia communications services over packet data networks, such as the Internet, continues to grow. Consequently, the demand for higher capacity in core data transport networks continues to grow. For service providers, core data transport networks are typically optical networks based on fiber optic technology. To meet the ever-growing capacity demand, 100 G/s per channel data rate operating at 2 bit/s/Hz spectral efficiency (SE) by using quadrature phase shift keying in the pulse-density (PDM-QPSK) modulation and digital signal processing (DSP) enabled coherent detection has been commercially deployed in the existing core networks. Also, research for the next generation of transport systems operating at even higher data rate (likely at 400 Gb/s or above) and higher SE by using more spectrally efficient high-order modulation formats (such as the well known quadrature amplitude modulation (QAM)) is underway. However, recent experimental results have revealed that it is extremely challenging to achieve long-reach transmission using high-order modulation formats because they are more vulnerable to various transmission impairments such as fiber nonlinear effects, laser phase noise, and amplifier noise.
To address the challenge caused by fiber nonlinear effects, several digital nonlinear compensation methods have been proposed, including the digital backward-propagation (DBP) based methods and a Volterra-based nonlinear equalization method. However, the implementation complexity of these digital methods is prohibitively high, making it almost impossible for them to be realized for any practical high-speed transmission systems. Several mid-link phase-conjugation based methods have also been proposed. However, these methods generally work well only for point-to-point submarine systems or specially designed super-channel systems. For typical terrestrial optical networks where reconfigurable optical add/drop multiplexers (ROADMs) are used to route optical wavelengths and different wavelength channels usually end up at different locations, there is still no feasible solution to mitigate fiber nonlinear effects for high-speed coherent optical transmission systems.
To reduce the impact of laser phase noise on system performance, several single-tap phase rotation filter (with fast adaptive rate) based phase recovery methods have been proposed. However, these methods only work well for systems without using long-memory equalizer at the receiver, i.e. the short-reach system or long-reach system using inline optical dispersion compensation. Because the use of inline optical dispersion compensation not only increases the complexity of the inline optical amplifier design, but also significantly reduces fiber nonlinear tolerance, purely electrical/digital dispersion compensation is usually required in a high-speed coherent optical transmission system. For such a communication system, a linear digital filter/equalizer with very long memory length has to be introduced at the receiver to compensate for the accumulated dispersion from the transmission fiber.